Ensure that you have your CallbackURL and ReturnURL ready to receive responses from A1Pay. Once a transaction is successful, A1Pay will redirect the user to your ReturnURL with URL parameter Ref which is the A1Pay transaction reference along with URL parameter Response which will be 00 if the transaction is successful.
Another URL parameter MerchantRef will also be passed to your ReturnURL. This will hold your unique transaction reference, and you can query A1Pay with this parameter. See more on this here.
A1Pay will also send a post request to your CallbackURL with the transaction status
Always ensure to call the A1Pay Query secure endpoint to confirm the transaction status received by the ReturnURL, this is the recommended practice to prevent malicious users from circumventing your system.